The adjustment and maintenance of solution pH with alkali as required for selective sulfide precipitation of zinc at a pH of 4.1 is known to improve both the hydrogen sulfide uptake in solution and the associated precipitation kinetics. The adjustment of pH also results in the co-precipitation of gypsum and other metals such as aluminum as aluminum hydroxide and reduces the purity and value of the recovered zinc sulfide product.
The precipitation of heavy metals as metallic sulfides from aqueous solutions is a common and long practiced method of solution purification and metal recovery. It is known that these precipitation reactions occur at a pH that is characteristic to the sulfide formed. The order in which the heavy metal sulfides precipitate with increasing pH is: As2S5, HgS, CuS, Sb2S3, Bi2S3, SnS2, CdS, PbS, SnS, ZnS, CoS, NiS, FeS, and MnS. See U.S. Pat. No. 4,278,539.
Hydrogen sulfide, ammonium sulfide, sodium sulfide, and sodium hydrosulfide (NaSH) are the most common precipitating agents. Typically, hydrogen sulfide gas is employed to react with the solubilized metal salts (usually sulfates and/or chlorides). The usual practice is to mix the metal containing feed solution with hydrogen sulfide in an amount in excess to that required to precipitate the metal sulfide of interest. Because pH is viewed as a crucial control step the pH of the aqueous feed liquid must be below that at which precipitation can be initiated. In the case of those metal sulfides, e.g., SnS2 and CdS, which form under slightly acidic conditions, it may be necessary to lower the pH, by adding an appropriate amount of an acid, to ensure that precipitation of non target elements is not initiated.
To recover metal sulfides such as As2S5, HgS, and CuS, which form even under very acidic conditions, a soluble salt of a metal (such as iron or manganese) the sulfide of which is soluble within the pH range of the feed liquid can be added to the solution. When hydrogen sulfide is introduced it reacts preferentially with the added metal ions to form soluble sulfides rather than with the metal ion or ions to be removed. When pH of the solution is increased, the sulfide ions are controllably precipitated as insoluble sulfides and isolated from the soluble sulfides. See U.S. Pat. No. 4,278,539.
Metals such as copper, nickel, cobalt, and zinc can be precipitated from process solutions as sulfides when their concentrations in solution are relatively low. This process is used as a solution purification or effluent treatment as well as for metal recovery. It has been reported that metal values at much higher concentrations can be isolated in this manner. For example, Outokumpu purifies a solution containing 20 g/L cobalt, 6-8 g/L nickel, 7-8 g/L copper, and 10-12 g/L zinc by selectively removing copper, zinc, nickel, and cobalt, in that order. See Textbook of Hydrometallurgy 2nd edition, Fathi Habashi.
Precipitation of the metal sulfide occurs by the following reaction:MSO4+H2S=>MS+H2SO4  (1)Acid is generated during the sulfide precipitation, and continuous addition of alkali to control the pH within the optimal range is normally practiced. Lime neutralization of this acid results in the contamination of the zinc product by gypsum, according to the following reaction:H2SO4+Ca(OH)2=>3CaSO4.2H2O  (1)
At the pH required for zinc precipitation by sulfide, the following reactions may also occur upon adjustment, if aluminum is present in solution:Al2(SO4)3+6NaOH=>2Al(OH)3+3Na2SO4  (1)Al2(SO4)3+3Ca(OH)2+6H2O=>2Al(OH)3+3CaSO4.2H2O  (2)Use of lime results in gypsum and aluminum hydroxide formation. Employing sodium hydroxide produces soluble sodium sulfate, which reduces the contamination of the zinc product recovered to some degree.